The swine flu epidemic crossed new borders Tuesday with the first cases confirmed in the Middle East and the Asia-Pacific region, as world health officials said they suspect American patients may have transmitted the virus to others in the U.S.

Most people confirmed with the new swine flu were infected in Mexico, where the number of deaths blamed on the virus has surpassed 150.

But confirmation that people have been infecting others in locations outside Mexico would indicate that the disease was spreading beyond travelers returning from Mexico, World Health Organization spokesman Gregory Hartl told reporters on Tuesday in Geneva.

Hartl said the source of some infections in the United States, Canada and Britain was unclear.

The swine flu has already spread to at least six countries besides Mexico, prompting WHO to raise its alert level on Monday but not call for travel bans or border closings. On Tuesday, countries, including Canada, Israel and France, warned their citizens to avoid nonessential travel to Mexico.

“Border controls do not work. Travel restrictions do not work,” Hartl said, recalling the 2003 SARS epidemic that killed 774 people, mostly in Asia, and slowed the global economy. “There was much more economic disruption caused by these measures than there was public health benefit.”

Hartl said WHO is advising countries to provide sick people with treatments such as Tamiflu, and make sure national plans are in place to ease the impact of a larger outbreak.

“Governments will need to start thinking about larger-scale care for a specific disease in accident and emergency wards,” he said. “Do they have the infrastructure? Do they have the equipment? Do they have the medicines? This is the time, now, to prepare.”

WHO raised the alert level to Phase 4, meaning there is sustained human-to-human transmission causing outbreaks in at least one country. WHO’s pandemic alert system was revised after bird flu in Asia began to spread in 2004. Monday was the first time it has ever been raised above Phase 3.

Flu deaths are nothing new in the United States or elsewhere. The Atlanta-based Centers for Disease Control and Prevention estimates that about 36,000 people died of flu-related causes each year, on average, during the 1990s in the United States.

But the new flu strain is a combination of pig, bird and human viruses that humans may have no natural immunity to.

The city was evaluating whether to keep open a subway system that provides 5 million trips a day.

New Zealand reported Tuesday that 11 people who recently returned from Mexico contracted the virus. Tests conducted at a WHO laboratory in Australia had confirmed three cases of swine flu among 11 members of the group who were showing symptoms, New Zealand Health Minister Tony Ryall said.

Officials decided that was evidence enough to assume the whole group was infected, he said.

Israel’s Health Ministry confirmed Tuesday the region’s first swine flu case in the city of Netanya. The patient, 26, recently returned from Mexico and had contracted it. A hospital official said the patient had recovered, but will remain hospitalized until the health ministry approves his release.

A hospital in the central town of Kfar Saba said a 47-year-old man who returned from Mexico two days ago and checked himself in when he felt ill had the strain and was being kept in isolation. He was expected to recover.

Meanwhile, a second case was confirmed Tuesday in Spain, Health Minister Trinidad Jimenez said, a day after the country reported its first case. The 23-year-old student, one of 26 patients under observation, was not in serious condition, Jimenez said.

With the virus spreading, the U.S. stepped up checks of people entering the country and warned Americans to avoid nonessential travel to Mexico.

“We anticipate that there will be confirmed cases in more states as we go through the coming days,” Homeland Security Secretary Janet Napolitano said on NBC’s “Today” show on Tuesday.

The Food and Drug Administration late Monday issued emergency guidance that allows certain antiviral drugs to be used in a broader range of the population in case mass dosing is needed to deal with an outbreak.

Mexico, where the number of deaths believed caused by swine flu rose by 50 percent on Monday to 152, is suspected to be the center of the outbreak. But Mexican Health Secretary Jose Angel Cordova late Monday said no one knows where the outbreak began, and implied it may have started in the U.S.

“I think it is very risky to say, or want to say, what the point of origin or dissemination of it is, given that there had already been cases reported in southern California and Texas,” Cordova told a press conference.

Mexico City Health Secretary Armando Ahued said three people died in the capital Monday, but it was unclear if they were included in the national toll. He said 6,610 people went to city hospitals Monday with flu symptoms, but only 29 were remained hospitalized.

Dr. Nancy Cox of the CDC has said she believes the earliest onset of swine flu in the U.S. was on March 28. Cordova said a sample taken from a 4-year-old boy in Mexico’s Veracruz state in early April tested positive for swine flu. However, it is not known when the boy, who later recovered, became infected.

A decision by WHO to put an alert at Phases 4 or 5 signals that the virus is becoming increasingly adept at spreading among humans. Phase 6 is for a full-blown pandemic, characterized by outbreaks in at least two regions of the world.

Fifty cases — none fatal and most of them mild — were confirmed in the United States. Including the New Zealand, Israeli and new Spanish reports, there were 92 confirmed cases worldwide on Tuesday. That included six in Canada, one in Spain and two in Scotland.

Amid the alarm, there was a spot of good news. The number of new cases reported by Mexico’s largest government hospitals has been declining the past three days, Cordova said, from 141 on Saturday to 119 on Sunday and 110 Monday.

Symptoms include a fever of more than 100, coughing, joint aches, severe headache and, in some cases, vomiting and diarrhea. Many victims have been in their 30s and 40s — not the very old or young who typically succumb to the flu.

So far, no deaths from the new virus have been reported outside Mexico.

It could take four to six months before the first batch of vaccines are available, WHO said. Some antiflu drugs do work once someone is sick.

The best way to keep the disease from spreading, the CDC’s acting director, Richard Besser, said, is by taking everyday precautions such as frequent handwashing, covering up coughs and sneezes, and staying away from work or school if not feeling well.

Russia, Hong Kong and Taiwan said they would quarantine visitors showing symptoms of the virus.

World stock markets fell Tuesday as investors worried that any swine flu pandemic could derail a global economic recovery. In the U.S., stocks fell moderately in early trading as investors worried that a growth in swine flu cases could hurt industries such as travel and tourism.

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AP writers Mark Stevenson in Mexico City, Mike Stobbe in Atlanta, Ray Lilley in Wellington, New Zealand, Aron Heller in Jerusalem, Frank Jordans and Sandy Higgins in Geneva, Aron Heller in Jerusalem, Maria Cheng in London and Pan Pylas in London contributed to this report.